Conventional silver halide emulsion layers contain grains which are randomly dispersed with respect to one another. This random distribution of silver halide grains on a projected area basis is non-uniform, which may give rise to undesirable variation in image density which is a significant source of image noise in photographic systems.
Contributions to the undesirable variation in image density of a photographic image come from many sources. Principally, the random nature of the absorption of the light quanta by the individual photoreceptors, e.g., silver halide grains; variations in the final effect on the image visual density of each photoreceptor; and the random spatial distribution of the photoreceptors. Discussions of such effects are available in standard texts, in particular, Image Science, by J. D. Dainty and R. Shaw, Academic Press (1974), Chapters 1 and 3.
The desirability of forming photosensitive silver halide emulsions wherein the photosensitive grains are in a predetermined spaced array, that is, the grains are spaced symmetrically and apart from each other, has long existed in the art. A regular array of photoreceptors would eliminate the last of the above-mentioned contributions. It would have a greater effect as more of the photoreceptors participate in the image formation. When all photoreceptors contribute to the image, a regular array would produce no array-based density variation, whereas a random array produces a root mean square (RMS) image density variation proportional to the image density of the array. For conventional photographic systems this effect will be maximum at high negative densities, i.e., the more exposed parts of the image. For diffusion transfer positive images, this effect will be maximum at high positive densities; i.e., the less exposed parts of the image. The maximum contribution of the array variations could be reduced to substantially no macroscopic variation in these regions by using a regular array of photoreceptors.
The reduction in the variation in image density is less but still significant in intermediate exposure regions. A model calculation by G. Langner in "Zur Berechnung der Kornigkeit und der Kontrastschwelle photographischer Schichten" in Photogr. Korres. 99, 177 (1963) (see also p. 103 of Dainty and Shaw, above) comparing the theoretical means square variation in density for identical grains distributed randomly and regularly shows that this variation will be maximum in a regular array at half of the maximum density, where it will be only half of the variation expected in a random array of similar grains. At all density levels above zero the regular array would have a lower variation than the random array, although this difference would become small at low densities.
For a product in which all photoreceptors contribute to the image density at all density levels, for example, a transparency in which both the positive and negative images remain, see for example, U.S. Pat. No. 3,894,871, issued July 15, 1975 to Edwin H. Land, the variation in image density from a regular array would be significantly less than that from a random array at all density levels.
It will also be seen that the problem of clumping or agglomeration of silver halide grains would be virtually eliminated by a regular array of grains.
U.S. Pat. No. 3,019,124 is directed to a method of manufacturing photographic elements having at least two sets of submacroscopic elemental portions systematically arranged in a screen pattern. This method comprises applying a first light sensitive layer in a uniform thickness to a support, embossing the coated layer to form a relief impression having systematically arranged spaced-apart elevated surface sections joined by depressed sections interspersed therebetween, and applying a second light sensitive layer having a different spectral sensitivity to fill the depressions remaining in the surface to the level of the raised sections. This embossing and coating may be repeated to provide third areas of different sensitivity. While the resulting array of photosensitive areas is regular, the silver halide grains still are randomly arrayed within each photosensitive area.